1. Field of Invention
The invention relates to optical devices to compensate for visual defects, particularly visual field loss.
2. Prior Art
Individuals who have suffered a cerebrovascular accident (CVA) or traumatic brain injury (TBI) sometimes incur a homonymous hemianopsia (visual field loss) that affects the same visual field projected from either eye. For example, an individual with a neurological event affecting the left cerebral hemisphere may incur a right homonymous hemianopsia. This means that the right visual field of both eyes will be obliterated due to a loss of sensory function in the visual system. It causes difficulty fixating on objects and people because half of the object or person will be missing. For example, an individual will only see half of a person's face when aiming both eyes directly at the person before them.
A visual field loss also causes a visual midline shift syndrome which has been documented in literature (Visual Midline Shift Syndrome, William Padula and Stephanie Argyris (1996)). A visual midline shift syndrome usually shifts opposite the side of the visual field loss causing problems with balance and orientation in space. Treatment with yoked prisms has been found effective for shifting the midline to a more centered position, thereby increasing weight bearing to the affected side and improving posture and balance. The visual field loss, however, remains a functional interference as well as an issue of safety. Objects on the side of the field loss will be totally undetectable.
Enhanced field prism systems have been used effectively to improve awareness of objects and space in the affected field. To utilize these prisms, a prism system is placed embedded or onto the eyeglass lenses. For an individual with a right homonymous hemianopsia, one or two prisms is applied to the lens or lenses (“Field Expansion for Homonymous Hemiaopsia by Optically Induced Peripheral Exotropsia”, Optometry and Vision Science, Eli Peli, DO, Vol. 77, No 9, PP 453-464 and “Vision Aids for People Having Homonymous Hemianopsia” by the National Institute for Rehabilitation Engineering (1987)). If a monocular system is being temporal designed, the prism is placed base-out toward the temporal side of the lens with the apex aligning just temporal to the line of sight of the right eye. When an individual looks straight ahead, they will not see the prism until they shift or scan to the right and the line of sight is bisected by the prism. The prism will shift an image in the right field in a more nasal direction so that objects through the prism appear more directly in front of the person. Upon removing the prism glasses, the individual will find that the object is further to the right. If the device is used binocularly, two prisms are placed; one on each lens. For the left lens, the base-out end of the prism is placed on the nasal border of the lens with the apex of the prism aligned just nasally to the line of sight of the left eye. When individuals have a homonymous hemianopsia and attempt to read, they will have significant difficulties since their field loss often interferes with their ability to shift their eyes. For an individual with a right homonymous hemianopsia, it is as if they are always attempting to read into the blind field. Many of these individuals only see several letters or one word at a time. It makes reading tedious and frustrating. Individuals with a left homonymous hemianopsia will often be able to read across a line of print, but due to a field loss on the left side will have difficulty finding the next line of print.
Individuals who are utilizing the enhanced field prism system for ambulation and navigation will still often miss objects in their periphery if their scan is not complete.